Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads from worm gears to traction drives and do so over a wider range of operating conditions than mineral oil lubricants.
In accordance with customary practice in the lubricant arts, PAO's have been blended with a variety of additives such as functional chemicals, oligomers and high polymers and other synthetic and mineral oil based lubricants to confer or improve upon lubricant properties necessary for applications such as engine lubricants, hydraulic fluids, gear lubricants, etc. Blends and their additive components are described in Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526, incorporated herein in its entirety by reference. A particular goal in the formulation of blends is the enhancement of viscosity index (VI) by the addition of VI improvers which are typically high molecular weight synthetic organic molecules. Such additives are commonly produced from polyisobutylenes, polymethacrylates and polyalkylstyrenes, and used in the molecular weight range of about 45,000 to about 1,700,000. While effective in improving viscosity index, these VI improvers have been found to be deficient in that the very property of high molecular weight that makes them useful as VI improvers also confers upon the blend a vulnerability in shear stability during actual applications. This deficiency dramatically reduces the range of usefulness applications for many VI improver additives. VI enhancers more frequently used are high molecular weight acrylics. Their usefulness is further compromised by cost since they are relatively expensive polymeric substances that may constitute a significant proportion of the final lubricant blend. Accordingly, workers in the lubricant arts continue to search for additives to produce better lubricant blends with high viscosity index. However, VI improvers and lubricant mixtures containing VI improvers are preferred that are less vulnerable to viscosity degradation by shearing forces in actual applications. Preferred liquids are those that exhibit Newtonian behavior under conditions of high temperature and high shear rate, i.e., viscosities which are independent of shear rate.
Recently, novel lubricant compositions (referred to herein as HVI-PAO and the HVI-PAO process) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. patent applications Ser. No. 210,434 and 210,435 filed Jun. 23, 1988, now U.S. Pat. Nos. 4,827,064 and 4,827,023 to M. Wu, incorporated herein by reference in their entirety. The process comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having branch ratios of less than 0.19 and pour point below -15 .degree. C. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalpha-olefins using Lewis acid catalyst. Their very unique structure provides opportunities for the formulation of superior lubricant blends.
Considering the abundance of C.sub.2 to C.sub.5 alpha-olefins in the petroleum refinery, and their low cost, it has long been been recognized that they could be a preferred source of low cost lubricant if they could be oligomerized to provide high viscosity index lubricant in good yield with a manageable, regenerable, non-corrosive catalyst such as reduced chromium on porous support as taught in the foregoing patents to M. Wu. These objectives are taught and reached in the process and compositions of U.S. Pat. No. 4,990,709. The products of the process taught in U.S. Pat. 4,990,709 exhibit a very unique structure that confers upon the products the properties of novel compositions. In conventional Ziegler oligomerization of alpha olefins it is well known in the art that the oligomers produced contain a high degree of structural regularity, or regio-regularity, as exhibited by a preponderance of head-to-tail bonding in the oligomerization of these alpha olefins. In the products from Ziegler catalyzed oligomerization not more than twenty percent of the repeating units are linked by head-to-head and tail-to-tail bonding. In the olefin oligomers produced from the reduced metal oxide catalysts taught in the patents to M. Wu it has been found that at least forty percent of the repeating units are bonded by head-to-head or tail-to-tail connections. The oligomers contain not more than 60% regio-regularity, where 100% regio-regularity corresponds with all head-to-tail connections for the recurring oligomeric unit. At least twenty percent of the repeating units are bonded by irregular head-to-head or tail-to-tail connections. These oligomers have a regio-irregularity of at least twenty percent, usually from 20 to 40 percent, and in most cases, not more than 60 percent.
Accordingly, it is an object of the present invention to provide novel, low viscosity lubricant VI improver compositions having high viscosity index and shear stability from alpha-olefins.
It is a further object of the present invention to provide novel lubricant basestock blends from low viscosity, high viscosity index C.sub.2 -C.sub.5 copolymers or homopolymers in conjunction with synthetic and natural petroleum lubricant.